Ruby-throated Hummingbirds, Small but Mighty

September 2016 

by Linda Martinson 

It is time to say Bon Voyage to our ruby-throated hummingbirds, those remarkable, singular, and curious birds, as they begin their annual migration back to Mexico and Central America for the winter. The ruby-throated hummingbird (Archilochus columbris) is native to the Appalachians and the only breeding hummingbird species along the East Coast of North America. It is by far the most common species east of the Mississippi River. 

They are tiny birds, averaging 3 to 4 inches in wing span and in length, including its long, slender beak, and weighing only about .12 to .13 ounces/2.5 to 3 grams. To put this in perspective, the average ruby-throated hummingbird weighs about as much as two or three paperclips. If you could squeeze them into the right-sized envelope, you could mail 9 or 10 hummingbirds with just one Forever stamp! 

Most species of birds have made an evolutionary trade-off: to achieve flight, they gave up teeth and flexible skeletal extremities and developed a mostly stiffened, but lighter weight and much more compact frame with a large sternum and powerful pectoral or flight muscles. They also have a more demanding metabolism with higher body temperatures, heart rates, and oxygen consumption. Birds have to eat relatively much more food for their body weight than other animals to meet these higher metabolic requirements of flying. Birds also have better vision and keener hearing than most other mammals, and all birds see in color.

Even by bird standards, hummingbirds are elite athletes, flying almost constantly with remarkable strength, speed, precision, and agility. Most birds have pectoral muscles that make up about 20 percent of their body weight. In contrast, about 25 – 30% of the body weight of hummingbirds is pectoral muscle with additional skeletal and muscular adaptations that give them greater flight capability and make them unique among birds. Unlike the wings of other birds that connect along the sides of their bodies and generate power only on the downstroke, hummingbirds have long, blade-like wings that connect to their bodies from the shoulder joint. This allows their wings to rotate almost 180 degrees so that they can fly more like insects, i.e., up and down and backward as well as forward, and are able to hover in mid-air with exceptional precision and efficiency. 

Hummingbirds are similar to insects in another way, too. Because they have a long slender beak and extendable tongue and therefore can sip nectar from flowers, hummingbirds are a key pollinator in the continual United States. Many of our native wildflowers in the Southern Appalachians (e.g., jewelweed and cardinal flowers) have adapted to, and depend upon, pollination by hummingbirds.

 Pollination in action    Photo credit: Jim Poling, Certified Blue Ridge Naturalist

There are trade-offs to hummingbird uniqueness among birds. One is that, although hummingbirds can perch and sidestep along a twig for a short distance, they cannot walk like other birds. Also, hummingbirds have among the highest metabolic rates of any animal. At rest, a hummingbird’s heart beats about 615 times each minute, and it breathes at about 250 breaths per minute. During flight, a hummingbird beats its wings about 70 times per second, fast enough to make a buzzing sounds, and its heart rate doubles up to 1260 beats per minute. A hummingbird in flight has an oxygen consumption rate about 10 times higher than that recorded for elite human athletes, and its hovering capability is considered more efficient than most helicopters and drones. Finally, hummingbirds have to eat almost constantly to support their high metabolism and the caloric demands of their athletic lives. One estimate is that every day they have to eat up to twice their weight in food, primarily nectar from flowers and small insects which they often nab in flight. If temperatures drop during the day and on cold nights, a hummingbird can enter a hypothermic torpor to conserve energy, lowering its body temperature significantly and reducing its metabolic rate by as much as 95%.

Ruby-throated hummingbird aerodynamics  Photo credit: Dan Pancano

Hummingbirds are feisty and solitary. They don’t flock or form nesting pairs, and both males and females are territorial and aggressive toward all other birds, including hummingbirds, and other perceived threats. Males usually vigorously defend feeding sites and females, their nesting sites. They are together only to mate, and that just takes a few minutes. Male hummingbirds entice females with their bright red throats, arcing horizontal dives, and clicking noises, then they mate quickly and fly off, hopefully to another tryst. Their breeding habitat is in both deciduous and pine forests, along the edges of wood areas, and in orchards and gardens. The females, who are grayish-green without a ruby throat, build a small and usually well camouflaged nest with a diameter about the size of a half dollar coin on a small tree limb. 

Male ruby-throated hummingbird  Photo credit: Laura Erickson

A hummingbird nest is cozy: bound with spider web silk so it is stretches as the baby birds hatch and grow, with plant down or animal fur on the inside and lichen on the outside. The mama bird lays usually two eggs, each about the size of a navy bean, and broods them for 12 to 14 days. She feeds the hatched chicks 1 – 3 times every hour by regurgitating a mixture of mostly small insects and spiders with a little nectar into their beaks. The nestlings make their first flight when they are about 18 – 22 days old. Only the females care for the chicks, and they may have more than one clutch per summer. Hummingbirds are too nimble and quick to have many predators, but raptors can catch them and they are vulnerable when they are chicks, nesting, sleeping, or in a torpor. Then they can be captured more easily by blue jays, lizards, snakes, feral cats, and raccoons, for example, and even by praying mantises and large spiders.

Hummingbird eggs in nest       Photo credit: Karen Szilat

It’s hard to conceive of these tiny birds migrating up to 2,000 miles, from as far away as Central America to Canada, twice a year. But most of them do, and they fly alone the whole way including a nonstop flight of 500+ miles across the Gulf of Mexico. As much as they can, the birds fly low along the coast line just above the tree tops for safety and to catch some food. They bulk up before they have to set off across the Gulf, sometimes to double their body weight, and then they fly across the water non-stop for about 20 hours.  

Hummingbirds are small but mighty, and they are also quite intelligent in terms of avian cognition — they recognize hummingbird feeders and remember their locations, and many of their devoted human benefactors swear that the birds recognize them, too. Like many other species of birds, hummingbirds have become increasingly dependent on feeders as more and more land has been developed. If you do feed hummingbirds, use about 4 to 1 sugar to water mixture and don’t add red dye because it may be toxic to them. Check migration schedules and sightings on the Web, and put your feeder out before they are scheduled to arrive and leave it up after they start to migrate in order to support both the early birds and the stragglers.

Credit: JIm Poling

References:

blog.lauraerickson.com

Frick, Jennifer. Mountain Nature. University of North Carolina Press, 2010.

“Ruby-throated Hummingbirds, Life History, All About Birds – Cornell Lab of Ornithology” allaboutbirds.org